Hi,
It's a bit more than we could probably explain right here, but there's a section in Help (Creating Rollovers and Animations (ImageReady)) that should get you going. To be honest though, I do most of my slicing with Fireworks MX now. I create my pages in Photoshop but then suck them into Fireworks to do the cutting up for the rollovers, and the 'dis-jointed' rollovers like when the image changes at the bottom. There's a pretty good tutorial or two in FW that will show you how to do most of it. It's pretty quick and I find Fireworks to be a lot better for prepping that kind of stuff for the web. If you don't have it, check macromedia.com for a fully working trial copy. You'll love it
.
All the best,
E3